Gunfire erupted in downtown Oakland near a vigil for two drowning victims, leaving a 16-year-old girl dead and sending people running for cover in the days after the worst mass shooting in modern U.S. history.

A Pakistani woman's role as a shooter in the San Bernardino massacre is raising fears that foreign-born brides who support the Islamic State group could marry Americans to come to the U.S. to carry out extremist missions.

Islamic State group propaganda is resonating with a growing number of young women and teenagers, complicating U.S. counterterrorism efforts to identify and monitor supporters such as Tashfeen Malik, the 29-year-old mother suspected in the California shootings along with her husband.

At a church, a mosque, a makeshift street-corner memorial and other sites, they gathered Sunday to mourn the 14 victims of the San Bernardino massacre and lament that the community has now been added to the tragic list of U.S. cities scarred by terrible violence.

Since the attack Wednesday at a social service centre in Southern California, the state's strict laws and the apparent legal purchase of the weapons have set off a debate over the effectiveness of gun measures and whether getting tougher would help prevent more violence.

A change came over Tashfeen Malik two or three years ago. She started dressing more conservatively, wearing a scarf that covered nearly all of her face, and became more devout in her Muslim faith, according to some who knew her in Pakistan.

The announcement that one of the two suspects in the Southern California rampage that left 14 dead had pledged her allegiance to the Islamic State group provides an indication of a possible motive for the commando-style attack.